betta man....what "bad bacteria" are you talking about ??
yes ; carbon will remove chemicals and other impurities ; but only for a short period of time..although it is argued ; it is pretty much about 2-3 weeks..it then becomes saturated and no longer able to absorb any more.if left in the filter it will start to leech toxins back into ther tank after a while..
a filter will remove debris.....but it do provide a place for bacteria to build that will maintain the the levels of ammonia and nitrite....yes , the gravel in a tank will also do that.but the gravel by itself will not remove debris.thus the need for a filter...plants will help some , but are not enough to not use filtration..yes , you can keep unfiltered tanks , but one needs to be somewhat more experienced in fishkeeping.....

Personally I can't imagine someone not recommending a filter. I mean it just doesn't even make sense not to suggest one. They are beneficial and often outright important. A plant helps with nitrates. It does not help with all of the other junk that can float around in the tank. I keeps a stable source of good bacteria so you can do gravel vacuuming without worrying about harming your biology.

Seriously dude, get a filter.

Get a heater, they need heat as they are tropical. They make tiny ones for 1 gallon tanks that go under the gravel, the look like a little paddle. They are inexpensive.

In a one gallon- water change every 1-3 days. In a 5 gallon water change weekly. Keep in mind that just because you "can" be lazy and get away with longer between, does not mean you "should" as your tank is more vulnerable to negative consequences (such as crashing) with less than weekly water changes in a 5 gallon. You will want to change about 10-20% weekly.

i agree with obs...
there are a lot of things that one can "get away with" ; such as having 200 young common plecos in a 20 long...it all depends on how well one knows what they are doing...
but in general it is always best to make sure you do everything you can to make sure your fish thrive...know their needs and provide them..

I should clarify- I am not saying that the filter has to have carbon. If you are going to use carbon then change it every 3-4 weeks. You do not need carbon in most instances other than to pull out medications. I don't use carbon in my filters.

In the one gallon put in a small sponge filter. In the 5 you can go with sponge or a smaller hang on back.

well kid ; you are waaaay wrong...angelclown was talking about ammonia and nitrites...they are the "bad bacteria"...and filters remove them..gravel does not remove them , but houses the good bacteria that consumes them..just like the other hard surfaces in the tank..plants help some because they also provide a surface for the good bacteria to inhabit..
as i have been trying to tell folks all along...;carbon is not really needed all the time..mainly just for removing chemicals , impurities and staining...it is functional for only a short period of time...and later on it will leech toxins back into the tank...
why do manufacturers recommend carbon ? because it is more revenue for them.....
and that is why they tell you to change the filter cartridge every 2 weeks...

Why would you not correct someone for using the word bacteria for ammonia? They are not the same and should not be used as such. The nitrogen cycle and other aspects of aqurium keeping are hard enough to understand without confusing the terms.